A382072 Lexicographically earliest sequence of distinct positive integers such that for any n > 0, n*a(n) is a factorial number.

1, 3, 2, 6, 24, 4, 720, 15, 80, 12, 3628800, 10, 479001600, 360, 8, 45, 20922789888000, 40, 6402373705728000, 36, 240, 1814400, 1124000727777607680000, 5, 145152, 239500800, 13440, 180, 304888344611713860501504000000, 168, 265252859812191058636308480000000

Offset: 1

Author

Rémy Sigrist, Mar 14 2025

Keywords

• nonn

Comments

This sequence is a self-inverse permutation of the positive integers.

Examples

			The first terms, alongside n*a(n), are:
  n   a(n)       n*a(n)
  --  ---------  ------
   1          1      1!
   2          3      3!
   3          2      3!
   4          6      4!
   5         24      5!
   6          4      4!
   7        720      7!
   8         15      5!
   9         80      6!
  10         12      5!
  11    3628800     11!
  12         10      5!
  13  479001600     13!
  14        360      7!
  15          8      5!
		

Links

• Index entries for sequences that are permutations of the natural numbers

Crossrefs

Cf. A000142, A007672, A382067.